1. Field of the Invention
This invention relates generally to centrifuges, to controls for centrifuges, and in certain particular aspects to programmed media useful in such centrifuge control systems.
2. Description of Related Art
In general, a screw decanter centrifuge has a cylindrical bowl rotating in one direction and a screw conveyor disposed concentrically in the bowl and rotating in the same direction as that of the bowl with a differential speed. The bowl creates a centrifugal force to dehydrate a fluid feed mixture. It is rotated at a constant but variable speed to separate the feed mixture into a component containing solids (hereinafter called dehydrated cake) and other components (liquid). As a result of the centrifugal force created by this rotation, the solids which are heavier than water are collected on the inner wall of the bowl. The screw conveyor is rotated at a relative velocity slightly differentiated from the velocity of the bowl. This differential speed creates a relative motion between the series of screw and the bowl inner wall, which causes the solids to be conveyed slowly in the direction of the cylinder axis along the bowl inner wall. The light component or liquid in the feed mixture is separated from the solids due to the centrifugal force, and moves toward the inside in the radial direction. The dehydrated cake which is a separated heavy material, and the liquid which is a separated light material, are usually discharged separately from opposite ends of the bowl.
The differential speed between the screw conveyor and the bowl can be varied during the operation of the centrifuge dependent on several parameters and quality of the feed mixture to be taken out by separation. In actual operation these conditions are well-known factors. Accordingly, maintenance of constant revolutions is generally required for the bowl. On the other hand, regarding the number of revolutions of the screw conveyor, there are two systems, the first of which keeps the number of revolutions of the screw conveyor always constant in response to that of the bowl, and the second of which varies the number of revolutions of the screw conveyor in response to the carrying torque of the screw conveyor.
Many different industries use decanter centrifuges in varied applications. They are used in the oil industry to process drilling mud to separate undesired drilling solids from the liquid mud. Some decanter centrifuges, because of their continuous operation, have the advantage of being less susceptible to plugging by solids. Also, they may be shut down for long or short periods of time and then restarted with minimum difficulty, unlike certain centrifuges which require cleaning to remove dried solids. Often the solids/liquid mixture is processed at extraordinarily high feed rates. To accommodate such feed rates, high torques are encountered, much energy is required to process the mixture, and the physical size of the centrifuge can become enormous.
Various drive systems for creating a differential speed between the bowl and the screw of a centrifuge are available. One is a backdrive system for horizontal centrifuges which uses electric motors and a differential gear.
When such a centrifuge is used to process drilling material (drilling fluid with drilled cuttings therein), changing mud flow conditions often require a human operator to frequently adjust centrifuge motor speeds to optimize centrifuge treating performance. Often, centrifuges operate a compromise between high performance and long intervals between maintenance and repair operations. Problems can occur if the centrifuge's differential gearbox overheats or is damaged from too-high gearbox speed differentials. Gearbox damage and overheating can occur when the backdrive motor is operated in forward or in reverse. High speed differential settings can be important for efficient solids removal from drilling mud which contains an excess of drilled solids and silt. Both gearbox damage and centrifuge plugging should be avoided.
Centrifuge manufacturers often specify gearbox differential speeds that must not be exceeded if safe, efficient, optimal centrifuge operating life is to be achieved; but operators frequently do not manually adjust centrifuge speed differentials optimally, resulting in reduced centrifuge solids removal and/or shortened gearbox life. Centrifuge breakdowns due to non-optimal adjustment and/or operation outside of specified differential speed parameters in remote areas of oil and gas prospecting, and offshore, can be costly and cause expensive delays.
There is a need for a system that makes it easier for a bush human operator to adjust and maintain centrifuge operations at a balance between high performance and optimized gearbox life. There is a need for a system that prohibits damage to a centrifuge due to incorrect manual settings.